1. Field of the Invention
The present invention generally relates to a method for producing a permanent magnet as a composite of soft magnetic Fe.sub.3 B and hard magnetic R--Fe--B fine grains (in this specification, such a magnet will be called a "nanocomposite magnet"). More particularly, the present invention relates to: magnet material alloy for use in producing the nanocomposite magnet; powder of the magnet material alloy; methods for preparing the magnet material alloy; methods for preparing the nanocomposite magnet powder; method for producing the nanocomposite magnet; and motor including the nanocomposite magnet.
2. Description of the Related Art
In an Fe.sub.3 B/Nd.sub.2 Fe.sub.14 B nanocomposite magnet, soft magnetic Fe.sub.3 B and hard magnetic Nd.sub.2 Fe.sub.14 B crystallites are uniformly distributed and magnetically coupled to each other as a result of exchange interactions therebetween. Each of these crystallites is of a size on the order of several nanometers and such a magnet is a composite of these two types of crystalline phases. Thus, a magnet of this type is called a "nanocomposite magnet" with a "nanocomposite structure".
A nanocomposite magnet exhibits excellent magnetic properties; although the magnet contains the soft magnetic crystallites, those soft magnetic crystallites have been magnetically coupled to the hard magnetic ones. In addition, since the soft magnetic crystallites contained do not include any rare-earth elements such as neodymium (Nd), the total volume fraction of rare-earth elements is small. Accordingly, the magnet can be supplied constantly at a reduced production cost.
The nanocomposite magnet of this type is produced by rapidly quenching a molten material alloy to form an amorphous alloy and then heat-treating the amorphous alloy to generate the nanometer-scaled crystallites. In general, the amorphous alloy is produced by a melt-spinning technique such as a single roller method. According to the melt-spinning technique, a melt of a material alloy is injected through an orifice to the outer circumference of a rotating chill roller, e.g., a water cooling drum, to come into contact with the roller surface for just a short period of time, thereby rapidly cooling and solidifying the material alloy. In this method, the cooling rate is controllable by adjusting the surface velocity of the rotating chill roller, for example.
The alloy that has been solidified and detached from the chill roller, i.e., melt-spun alloy, is in the shape of a ribbon (or strip) elongated along the circumference of the roller. The ribbon of alloy gets crushed into flakes by a crusher and then pulverized into finer powder particles by a mechanical grinder.
Thereafter, the powder particles are heat-treated to crystallize. As a result, the Fe.sub.3 B and Nd.sub.2 Fe.sub.14 B crystallites are grown and magnetically coupled together through the exchange interactions.
The type of a metal structure resulting from the heat treatment plays a key role in improving the properties of the permanent magnet as a final product. The conventional heat treatment process, however, has various drawbacks in view of the controllability and reproducibility thereof. Specifically, since a large quantity of heat is generated in a short time during the crystallization of the amorphous material alloy, it is difficult for the heat treatment system to control the temperature of the processed alloy. If a great amount of material alloy powder were annealed at a time, in particular, then the temperature of the alloy powder would almost always be out of control. Thus, according to the conventional technique, the heat treatment should be performed on just a small amount of material powder at a time and the resultant processing rate (i.e., the amount of powder processable per unit time) is far from being satisfactory. Such a low processing rate constitutes a serious obstacle to mass-production of magnet powder.